The present invention relates to a linear motion guide bearing with a rail which is fabricated by using steel which is cold-drawn and induction-hardened. Particularly, in the rail, the track surface is harder than a conventional rail, wear resistance and cold working characteristic are improved, and service life of a die is increased.
Generally, the rail of a linear motion guide bearing is formed by using a steel bar which is formed by cold drawing and subjected to induction heating. A typical example of steel used for the formation of the rail is a JIS S53C steel and other similar graded steel, because of their good drawability. For example, according to "Linear Motion Guide Unit" mentioned in the magazine "Special Steel", February 1990, pp. 33 to 36 published by Special Steel Association of Japan, steel containing 0.53% C (carbon) and 1.5% Mn (manganese) by weight is employed for the rail of the linear motion guide bearing.
The hardness of the surface layer of steel which has been induction-hardened and tempered is proportional to the content of carbon of the steel. In the case of the aforementioned conventional steel, the content of carbon is relatively small. Hence, even if the heat treatment conditions of the steel are improved, the resultant surface layers vary widely in hardness, so that it is difficult to stably give a hardness higher than a Vickers hardness (H.sub.v) of 700 (corresponding to a Rockwell C hardness (H.sub.R C) of 60) to the surface layer.
On the other hand, as the content of carbon of steel increases, the cold working characteristics of the steel is adversely affected. Hence, in the cold-drawing operation, the service life of the die is lowered; that is, the cost for the die is increased, or the cold drawing speed must be decreased; and thus productivity of the cold-drawing process is lowered.